This invention relates to a method of producing a piston ring to be incorporated in an internal combustion engine, and more particularly, to a method of producing a steel piston ring such as a steel compression ring.
Internal combustion engines including those supercharged have been improved for high speed, while having a high compression ratio according to user's needs. Therefore, it is also required to improve the piston ring such as the compression ring incorporated in the internal combustion engine.
An unleaded fuel is widely used for the internal combustion engine because of an air pollution problem. However, a leaded fuel is still used for the internal combustion engine in many countries. In a cylinder of the internal combustion engine supplied with the leaded fuel oil, hydrochloric acid gas and sulfuric acid gas create a strongly corrosive atmosphere. Accordingly, in the conventional piston ring plated with a chrome, a chromium plating of a sliding surface of the conventional piston ring is remarkably worn away. To overcome this problem it has been known to plate thickly with chrome the piston ring to prevent the sliding surface from wearing. However, such a solution is costly.
An improvement in wear resistance and corrosion resistance of the piston ring is eagerly required at present. For this purpose, the sliding surface of the piston ring is subjected to a nitriding treatment.
FIG. 7 shows a part of a conventional compression ring 1 as a piston ring. As shown in FIG. 7, a surface of a steel base material 2 is provided with a nitrided layer 3 formed by a nitriding treatment. When the nitrided layer 3 is formed by nitriding, a very fragile porous layer called a "white layer" is formed on a surface portion of the nitrided layer 3. The fragile porous layer on the surface portion of the sliding surface 4 or on both side surfaces and the sliding surface 4 is removed during a successive steps, and then a product of the piston ring can be finished.
A first example of materials composing the steel base material 2 will be described hereunder. The materials are described by weight percent.
C (Carbon): 0.80-0.95
Si (Silicon): 0.35-0.50
Mn (Manganese): 0.25-0.40
Cr (Chromium): 17.00-18.00
Mo (Molybdenum): 1.00-1.25
V (Vanadium): 0.08-0.15
Fe (Iron): Remaining percent
Unavoidable impurity material: Trace
A second example of the materials is as follows.
C: 0.87-0.93
Si: 0.20-0.40
Mn: 0.20-0.40
Cr: 21.00-22.00
Mo: 0.20-0.40
Ni (Nickel): 0.90-1.10
Fe: Remaining percent
Unavoidable impurity material: Trace
The compression ring 1 is incorporated in the internal combustion engine, so as to be placed in a groove formed on a piston of the internal combustion engine. While the internal combustion engine is operated, the compression ring 1 in the piston's groove repeatedly expands and shrinks along a radial direction and repeatedly strikes against a wall of the groove. Accordingly, sometimes the nitrided layer 3 of the sliding surface 4 has cracks 5. If the cracks 5 grow, a part of the nitrided layer 3 flakes away thereby generating a flaking portion 6 on the sliding surface 4. This phenomenon causes a scuffing (extraordinary abrasion), and the compression ring 1 is broken in some cases.